We have examined human intestinal mononuclear cells and have found that they are capable of killing antibody coated chicken red blood cells but not antibody coated cell line cells. Furthermore, they respond well in allogeneic mixed leukocyte reactions but do not function as effector cells of cell mediated lympholysis. Finally, high spontaneous antibody synthesis with paradoxical suppression by pokeweed migoten has been observed for IgG, IgA, and IgM by both normal and ulcerative colitis intestinal mononuclear cells. Thus, intestinal mononuclear cells have a number of internaldichotomys and major differences from peripheral blood mononuclear cells. Investigation of the autologous mixed leukocyte reaction has revealed that macrophages and Null cells but not B cells will stimulate autologous T cells to proliferate and to generate suppressor cells while all three cell types stimulate in the allogeneic mixed leukocyte reaction. Thus there are differences between the antigens which stimulate autologous as opposed to allogeneic T cells. We have also observed that simple sugars will inhibit spontaneous but not antibody dependent cell mediated cytotoxicity, thus demonstrating that the two processes are potentially mediated by different receptors and that endogenous lectins may be involved in spontaneous cell mediated cytotoxicity.